During the early twentieth century, architects and builders experimented with the traditional mass masonry wall form and new materials on the market to build multi-component wall assemblies that prevented moisture from penetrating the exterior surface. These assemblies introduced the issue of differential durability to exterior building envelopes. By considering the different durabilities and vulnerabilities of materials in an assembly, heritage building professionals can act preemptively to anticipate potential deterioration between the interior and exterior surfaces of an assembly. This thesis develops a methodology using two examples of impermeable mass structural walls constructed in the first half of the twentieth century to consider differential durability and vulnerabilities in heritage buildings. It provides a preventive framework for practitioners to follow and stimulates thinking about potential causal factors and deterioration mechanisms behind a seemingly durable exterior surface. The proposed methodology identifies each material and its position in an assembly, assesses comparative durability of materials, identifies causal factors of deterioration and vulnerability of protective layers, and proposes interventions. While it has been developed for relatively simple exterior envelopes, it can be applied to modern buildings or be used when designing interventions and/or introducing new materials to an exterior assembly.